In the manufacture of powder metal forgings, such as an inner race of a constant velocity joint (CVJ), it is sometimes desirable to provide a through-hardened part directly from the forge press. This requires that the part be directly quenched after the part has been ejected from the forge tooling. In forging such a part, the upper die moves in a downward direction to the lower die to deform the billet, which forms the part. This results in flash forming on the sides of the part where the upper and lower dies meet, which is in an area of bearing races for an inner race of a CVJ. If the part is directly quenched, then the tool flash is in a hardened state. Although hard trimming, which is a method of shearing the flash from the part, is possible, it is not practical because the flash can exceed the hardness of the current trim creating a potentially dangerous situation for the operators and can also negatively impact the quality of the product. That is, the part can break apart during trimming and fly out of the confines of the tooling. Also, the bearing races are precision surfaces and fairly intricate so that they are not very amenable in general to shearing.
A method of forging a CVJ inner race is known whereby a segmented die (6 die segments) is used to form the CVJ inner race using a traditional cold forging technique. However, this technique requires a machine broach and a relatively long carburization process. Further, there are six vertical witness lines on the part corresponding to the six die segments. Other disadvantages of this method are that it is a relatively complex and expensive tooling arrangement, with a relatively short die life.
Additionally, CVJs are known which have alternating, or counter, ball-tracks, where shallow ends and deep ends of the tracks alternate position, i.e., which end they're at, on every other track. See for example U.S. Pat. No. 5,221,233. Such designs may be used in a constant velocity fixed joint for large articulation angles and a high torque capacity. One method of fabricating such devices is to use segmented dies, as described above, to be separated after the forging process. This adds complexity to the process, slower cycle times, and contamination of the sealing surfaces in a hot forging environment due to the die lubricants. Further, this process is used in cold forging processes to make CVJs.
In an inner race of a CVJ, with alternating ball tracks, shallow end and deep end of the ball tracks alternate every other track, and therefore the formed inner race has a lateral flow of material in the forging process. The process described above, wherein the upper die moves in a downward direction to the lower die to deform the billet, in addition to having flash forming on the sides of the part where the upper and lower dies meet, will not allow the die-set to separate after the part is formed due to the lateral flow of material during forging. It is possible to machine the CVJ with the alternating ball tracks out of bar-stock, or out of a powder metal part that does not have counter ball tracks, although this is expensive and material inefficient.
What is needed in the art is a design and process that gives a near-net shaped alternating ball race CVJ, or other powder metal parts which include lateral flow during the forging process, without sacrificing cycle time and maintaining a relative simplicity to the tool set.